COLUMBUS, Ohio (AP) — It’s been two decades this month since the longest deadly prison riot in U.S. history broke out in southern Ohio and there’s trepidation in the air.

A prisons chief in Colorado and a district attorney in Texas and his wife have been slain.

The ratio of inmates to guards inside Ohio’s prisons has crept up again after a dip that followed the 11-day siege at Lucasville’s Southern Ohio Correctional Facility in 1993.

Double-bunking inmates, a trigger in the uprising that left one corrections officer and nine inmates dead, is back in use at a prison in Toledo. Serious assaults requiring outside medical attention have jumped from an average of three per year to 16 last year, and gang membership, while down slightly, stands at 16 percent.

Paul Goldberg, past executive director of the Ohio Civil Service Employees Association, which represents unionized corrections officers, said “the red flags are there” that existed in 1993 but were ignored.

“It wasn’t until we actually had the death of (Corrections Officer) Bobby Vallandingham and the riot in Lucasville that people understood that we’d been serious and what we’d been saying was real,” Goldberg said. “I fear the same circumstances are emerging today.”

Vallandingham was among 12 staff members taken hostage on April 11, 1993, when inmates overtook the prison that sits 10 miles north of the Ohio River. They were exiting the recreation yard on an Easter Sunday when it happened. Vallandingham was killed on the fourth day of the occupation, after his inmate captors had flown a bed sheet out the windows threatening to kill a hostage if certain demands weren’t met.

Rioting inmates wanted to have single cells rather than be doubled up and wanted more classes and visitation. Muslim prisoners wanted an exemption from a mandatory tuberculosis test that they said violated their religion and an end to forced racial integration.

Historian-lawyers Staughton and Alice Lynd, a husband-and-wife team who have spent the past 20 years investigating circumstances surrounding the riot, are marking the anniversary with lectures around the state focusing on the five inmates sentenced to death for their roles in the riot.

Media access has never been allowed to the “Lucasville Five”: Siddique Abdullah Hasan (formerly Carlos Sanders), Jason Robb, George Skatzes, Namir Abdul Mateen (formerly James Were) and Keith LaMar. The Associated Press’ request to speak to them ahead of the Lucasville anniversary was denied.

Staughton Lynd, who has written a book asserting none of the five is Vallandingham’s killer, said the state has yet to accept its share of the responsibility in the uprising so that justice can be served and conditions improved.

The Lynds arranged for LaMar to speak by phone to about 60 participants at an April 3 event at Youngstown State University revisiting the riot. LaMar, who was convicted of having a role in the slaying of prisoner informants during the riot, discussed being held in solitary confinement for 17 years, Lynd said.

Ohio prisons director Gary Mohr authored a voluminous report on the causes of the Lucasville riot as director of then-Gov. George Voinovich’s Office of Criminal Justice. He said there’s no question safety and security have improved since then.

Mohr can tick off a laundry list of targeted programs, legislative efforts and infrastructure upgrades in the past 20 years — and even the past two — that are making prison conditions better and guards safer.

He said all maximum-security inmates are housed in single cells. Through technology, staff are in better communication and are able to manage inmates with minimal physical contact that can bring violence, he said. The state has installed 4,000 new security cameras and assembled special-response teams across the state trained to handle disturbances.

And the administration plans a bill stepping up sanctions against inmates who throw bodily fluids at guards, Mohr said.

Christopher Knecht, a former inmate at Lucasville who served time both during the riot and some years afterward, said the two eras can’t compare.

“The conditions now are nothing like they were,” he said. “The only complaints now would be issues dealing with guard-prisoner relationship, classification, property, food, visits and things of that nature — typical complaints found at all prisons.”

Yet the anniversary arrives as the national mood within the corrections profession is apprehensive.

Mohr considered slain Colorado prisons director Tom Clements a professional and personal friend. The two had talked a day before Clements was shot at his front door last month.

“Worrying is a sin, but I still worry,” said Mohr, who’s headed the Ohio Department of Rehabilitation and Correction since January 2011. “I think every director in this country is concerned about the safety and operations of the staff. We need to be. Just since I’ve been director, there have been seven corrections employees around the country that have lost their lives in the line of duty.”

Luke Van Sickle, president of the prison guards’ union at Lucasville, said the shadow of the riot is always present at the 1,625-acre prison, where 1,365 inmates are housed. That’s down about 500 inmates from 1993.

“You’ll constantly hear comments of ‘Well, we’re going to repeat ’93.’ They’ll whisper that as they go down the hallway and pass you,” he said. “As far as security, it’s business as usual (for the anniversary). But everyone’s on edge.”

Van Sickle said the deaths of Clements and North Texas District Attorney Mike McLelland and his wife, Cynthia, are combining this year with memories of the Sandy Hook Elementary School massacre in Connecticut that left 28 dead to raise tensions.

“That just proves that you’re not safe from inmates in a prison, and you’re no longer safe outside a prison,” he said.

He mentioned reduced staffing — including in Lucasville guard towers — and tougher qualifications for staff retirement as strains on the system. There’s also concern over a proposal to privatize Ohio’s prison food service and potentially cut back the volume or quality of meals.

Mohr said the Lucasville riot has taught him — and corrections officials across the U.S. — that prisons must combine tough sanctions against violence with opportunities for inmates to change. He said Ohio has added 526 beds for prisoners who commit violent acts as well as reintegration units that provide activities and education for those who display good behavior.

“We have to believe people can change,” he said. “We have to provide systems to provide positive reinforcement for positive change because, ultimately, 97 percent of the people are going to come back out and live in these communities, and we cannot return a more bitter, hostile, unprepared population to be citizens in Ohio.”